The present invention relates to a process for producing a flux for brazing, and more particularly to a process for producing a flux for brazing for use in fabricating automotive condensers, radiators, evaporators, heaters and other aluminum heat exchangers.
The percentages as used herein and in the appended claims are by weight, and the term "aluminum" is used herein as including alloys thereof.
In brazing aluminum materials, fluxes are used for removing oxide coatings from the surfaces to be joined together. While chloride fluxes have heretofore been used for this purpose, they leave residues which cause corrosion to aluminum after brazing and which must therefore be removed by washing. Nevertheless, depending on the construction of the brazed article, such residues are not always removable completely.
To overcome the above problem, U.S. Pat. No. 3,951,328 proposes a flux in the form of a mixture of potassium fluoaluminate complexes essentially free of unreacted KF and having a composition corresponding to an AlF.sub.3 /KF ratio between about 65:35 and about 45:55. The "potassium fluoaluminate complexes" refers to complexes of the type formed by fusion of AlF.sub.3 and KF, such complexes having the formulas K.sub.3 AlF.sub.6 and KAlF.sub.4. The complexes are formed by the fusion of AlF.sub.3 and KF to render the flux free from unreacted KF. When containing unreacted KF, the flux is hydroscopic and is unsuitable for use in the form of an aqueous slurry, since slurrying of the flux in water would result in solution of KF and consequent possibility of disproportionation of the flux on drying and melting point variability.
However, resorting to the step of fusing KF and AlF.sub.3 in forming the complexes entails an increase in the production cost of the flux and is unfavorable in view of production efficiency.